The MP2/6-31G** potential energy surface (PES) of the water molecule is used as a model to locate curves of valley ridge inÑection (VRI) points. Crossing points between VRI curves and approximations of the reaction path allow the identiÐcation of a symmetric bifurcation of the reaction path. The VRI points are calculated with the help of BraninÏs method. The recently proposed method of following the reduced gradient is used to calculate reaction path approximations of "" inversion ÏÏ and

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... ation together with their possible bifurcations. The results gained by a model PES (W. Quapp, M. Hirsch and D. Heidrich, T heor. Chem. Acc., 1998, 100, 285) are now conÐrmed by the ab initio surface of the water molecule. We discuss the chemical and physical meaning of VRI curves in terms of the fourfold cluster problem of highly excited rotational states ; the core area of our surface is of sufficient quality for this.